1. Field of the Invention
The present invention generally relates to controlling signal reception in television signal receivers, such as a digital television receiver, and more particularly, to an apparatus and method for detecting multipath signal effects in order to provide improved signal reception.
2. Background Information
Multipath signal effects, or simply multipath, can be a significant problem in wireless communication systems. Multipath occurs when two or more propagation paths exist between transmitting and receiving sites. A transmitted signal may arrive at the receiving site from multiple paths exhibiting various amounts of delay and attenuation. The multiple paths may result from reflections from man-made or natural structures, repeaters or the use of multiple transmitters.
The effect of multipath on conventional analog signals, such as National Television System Committee (NTSC) television signals, results in a ghost-like image horizontally displaced from the main image by an amount proportional to the reflected signal's delay. Multipath degradation is not visible in a digitally demodulated picture until a threshold is reached, resulting in a loss of demodulation lock. In a digital television signal receiver, uncorrected multipath introduces intersymbol interference (ISI) which increases the risk of decoding errors. The use of adaptive equalizers in the receiver can reduce the effects of multipath and improve system performance. However, multipath outside the time range of the adaptive equalizer is perceived as additional noise and causes degradation to the received signal-to-noise (S/N) ratio.
Conventional techniques for reducing the effects of multipath in digital television receivers generally attempt to operate upon a digital signal after it has been demodulated and converted into a digital bitstream. However, this type of technique has several problems. First, if the multipath effects are severe enough, it may not be possible to acquire a demodulation lock since carrier and timing signals can not be recovered. For example, in a digital television signal receiver utilizing vestigial sideband (VSB) demodulation, multipath effects may be such that the pilot carrier can not be recovered. Moreover, if there is a null on the side opposite the pilot carrier, timing recovery may not be possible.
Even in cases where carrier and timing signals can be recovered, this process inevitably takes longer when multipath is present. Accordingly, the time required to obtain a demodulation lock and perform signal equalization is longer when multipath is present. In view of these deficiencies, it is recognized herein that it is desirable to identify and rectify multipath signal effects prior to demodulation and equalization. The present invention addresses these and other issues.